In recent years, in the field of flexographic printing, a Computer to Plate Technique (CTP Technique), known as a digital image-forming technique, has been widely used as a general technique. The CTP technique is a method in which information processed on a computer is directly outputted onto a printing plate so that a concave/convex pattern to form a relief is obtained. This technique eliminates the necessity of negative film producing processes, thereby making it possible to reduce costs and time required for forming the negative film.
In the CTP technique, the negative film, conventionally used so as to cover areas that should not be polymerized, is replaced by a mask that is formed and integrated within a printing plate. As to a method for preparing this integrated mask, there has been proposed, for example, a method where an image mask is printed using an ink jet printer on a photosensitive plate (cf. Patent Document 1). According to this method however, there is a disadvantage that resolution is insufficient in a letterpress printing of high quality with medium tone.
Also, a method has been proposed where an infra-red ray sensitive layer that is opaque to chemical rays is formed on a photosensitive resin layer and by evaporating this infra-red ray sensitive layer by using an infrared laser, an image mask is formed (cf. Patent Document 2). This photosensitive printing original plate is constituted from photosensitive resin layer, barrier layer (protective layer) and infrared-sensitive layer and the barrier layer has a function of preventing the mass transfer between the photosensitive resin layer and the infrared-sensitive layer and of preventing the polymerization inhibition of the photosensitive resin layer due to oxygen in the air.
On the other hand, as to the photosensitive resin layer, that which is able to be developed by an aqueous type solution has been strongly demanded in view of affection to the environment and of workability. As to the flexographic photosensitive resin layer which is able to be developed by water, that comprising synthetic rubber and hydrophilic polymer has been put into the market (cf. Patent Document 3). Moreover, in recent years, there has been developed an aqueous developing plate where resolution is further enhanced by the use of latex which is present in fine grains as a main component (cf. Patent Document 4).
However, in the CTP plate using such a photosensitive resin layer which is able to be developed with water, the photosensitive resin is highly flexible and there is generated a problem caused thereby. To be more specific, there is a problem that wrinkles are formed on the whole plate and that reproducibility of fine lines is poor. They are generated when the plate is detached to return to the flat plane after the plate is attached to a drum in a laser processing. The above problem is particularly significant when a photosensitive resin layer comprising as a main component the latex is used. It is presumably because the latex is present in fine particles whereby the plate is more flexible.
As to a method for preventing the wrinkles on the plate, there has been proposed a constitution where a barrier layer (protective layer) on the photosensitive resin layer is omitted whereby the photosensitive resin layer directly contacts with the heat-sensitive mask layer (cf. Patent Document 5). However, when this method is applied to an aqueous developing plate comprising the latex as a main component, the polymerization inhibition of the photosensitive resin layer due to oxygen in the air is vigorous and satisfactory plate is never achieved. There has been also proposed a method where elastic modulus of the barrier layer (protective layer) on the photosensitive resin layer is made within a predetermined range (cf. Patent Document 6). In this method however, although it is effective for the wrinkles on the whole plate, no big improvement is achieved for reproducibility of fine lines. As such, it is the current status that no completely satisfactory plate has been achieved in an aqueous developing CTP plate or, particularly, an aqueous developing CTP comprising the latex as a main component.    Patent Document 1: German Patent No. 4117127    Patent Document 2: Japanese Patent Application Laid-Open (JP-A) No. 506201/95    Patent Document 3: Japanese Patent Application Laid-Open (JP-A) No. 198058/91    Patent Document 4: WO 2004/090638    Patent Document 5: Japanese Patent Application Laid-Open (JP-A) No. 305030/96    Patent Document 6: Japanese Patent Application Laid-Open (JP-A) No. 2006-3706